1. Field of the Invention
The present invention relates to interferometry and, in particular, to generating interference patterns having desired spatial characteristics. Such interference patterns are useful, for instance, for making Bragg gratings in optical fibers and for making phase masks for making Bragg gratings. This invention also relates to controlling the shape of a mirror in an interferometer so as to generate an interference pattern having desired spatial characteristics.
2. Description of the Related Art
A fiber Bragg grating is a series of refractive index fluctuations formed in an optical fiber. Such gratings are useful for carrying out spectrally selective operations such as filtering, spectral analysis, selective reflection, and selective coupling of light into and out of the fiber. Devices using fiber Bragg gratings are growing in importance. An interferometer may be used directly to generate the desired interference pattern in a photoresponsive fiber; alternatively, an interferometer may be used to generate a phase mask, and the phase mask used to generate the desired interference pattern in the manufacture of fiber Bragg gratings. The phase mask is typically made by exposing a photosensitive plate of glass to an interference pattern created by an interferometer. Illuminating a phase mask with one laser beam produces a pair of diffracted beams that are made to interfere, resulting in a series of interference fringes that fall on the optical fiber. Exposure of the fiber to these fringes leads to formation of the fiber Bragg grating.
There is a growing need for fiber Bragg gratings that are xe2x80x9cchirped,xe2x80x9d i.e., that have a spatially varying period, or a spatially varying separation between successive fringes. Chirped phase masks are readily used to make chirped fiber Bragg gratings. However, conventional interferometric methods are of limited value for making chirped phase masks. Typically, a curved mirror is inserted in the interferometer for this purpose. As a light beam is scanned across the curved surface of the mirror, the illuminated portion of the resulting interference pattern advances across the photosensitive plate while, at the same time, the period of the interference pattern changes. U.S. Pat. No. 5,363,239 issued to Mizrahi et al. provides a detailed background regarding the production of Bragg gratings in general and chirped Bragg gratings in particular, and describes interferometric methods including use of a curved mirror to generate spatially varying interference patterns. The curved mirrors conventionally used for this purpose are either rigid or are flexed by a controllable point load. In either case, the kinds of curvature profile available are very limited. As a consequence, chirp profiles obtainable in an end product, e.g., in fiber Bragg gratings, are of very limited kinds. In particular, it is difficult to provide an extremely linear chirp over a relatively long distance such as a distance greater than one centimeter.
The present invention provides new methods and apparatus for generating interference patterns that can be used to generate fiber Bragg gratings, phase masks, and the like with a broader range of available characteristics and improved ability to control those characteristics. In one aspect of the present invention, a reflecting apparatus is provided in which the shape of a mirror is controlled by forces of selected magnitude applied at a plurality of selected locations that are applied to the mirror through one or more rocker-arm assemblies so as to flex the mirror. In a preferred embodiment of this aspect, each rocker-arm sub-assembly is adjustable so that one reflecting apparatus may be configured to generate a wide variety of mirror shapes. In another aspect of the present invention, a reflecting apparatus having a mirror shape that is adjustable by a rocker-arm sub-assembly is disposed in an interferometer to provide a controllable or adjustable interference pattern. In a preferred embodiment of this aspect, the interferometer includes one such reflecting apparatus in the path of each of the interfering light beams.
In one embodiment, the present invention is an arrangement having a reflecting apparatus, the reflecting apparatus comprising a mirror, a support structure configured to support the mirror, and a mirror-flexing assembly configured to apply force to the supported mirror, wherein the mirror-flexing assembly comprises a rocker-arm sub-assembly comprising a rocker arm and two force-distribution members spaced apart on the rocker arm and configured to contact at two different locations on the supported mirror, and a force generator configured to generate a force that is pivotably applied to a pivot point of the rocker arm, wherein the force is applied through the force-distribution members to affect the shape of the supported mirror.
In another embodiment, the present invention is a method comprising the steps of supporting a mirror of a reflecting apparatus of an arrangement using a first support structure, and applying force to the supported mirror using a mirror-flexing assembly, wherein the mirror-flexing assembly comprises a rocker-arm sub-assembly comprising a rocker arm and two force-distribution members spaced apart on the rocker arm and configured to contact at two different locations on the supported mirror, and a force generator configured to generate a force that is pivotably applied to a pivot point of the rocker arm, wherein the force is applied through the force-distribution members to affect the shape of the supported mirror.